Dictyostelium: Cell Sorting and Patterning

Abstract

Multicellular development of the social amoeba Dictyostelium discoideum results from the chemotactic aggregation of single cells to form a fruiting body consisting of a stalk supporting a spore mass. The interplay of two extracellular signalling molecules, cAMP (cyclic adenosine monophosphate) and the chlorinated hexaphenone DIF‐1 (differentiation‐inducing factor 1), directs cellular differentiation, and cAMP is also the chemo‐attractant that orchestrates all of morphogenesis. Although this is a relatively unusual mode of pattern formation, most of the fundamental processes that typify development in higher organism are on display, and the genetic accessibility of the organism makes it a very powerful and relevant model system. It has proven particularly valuable for understanding the basic mechanism of chemotaxis and the role that chemotaxis can play in the morphogenesis of a multicellular structure.

Key concepts:

  • Dictyostelium cells aggregate by chemotaxis in response to propagating cAMP waves.

  • cAMP waves propagate through cAMP relay, the ability of the cells to detect a small cAMP signal, synthesis and secrete cAMP in response and thus pass the cAMP signal on to their neighbours.

  • Adaptation of the relay response ensures unidirectional wave propagation away from the centre.

  • Chemotactic cell movement involves detection of a cAMP gradient across the length of the cell, resulting in a polarization of actin–myosin cytoskeletal dynamics and movement up the gradient.

  • Multicellular morphogenesis, the arrangement of tissue in space and time, is controlled by the interplay of cAMP wave propagation and chemotaxis during all stages of development.

  • Dictyostelium development is separated from cell division but a cell's fate can be biased by its cell cycle position when development starts.

  • Prespore cells are formed in response to cAMP signalling.

  • Differentiation of prestalk cells is induced by a small organic molecule, DIF, which is secreted by the prespore cells. This induction forms part of a feedback loop that controls the prestalk–prespore ratio.

  • The tip‐organizer cells are a subset of the prestalk cells that differentiation in response to a high local cAMP concentation.

  • Dictyostelium pattern formation occurs by the sorting out of differentiated cell types.

Keywords: morphogenesis; chemotaxis; cell differentiation; cell sorting

Figure 1.

The Dictyostelium discoideum life cycle. In a clockwise order starting at the top: vegetative amoebae, darkfield waves, as observed during aggregation (they reflect the cells in different phases of the movement cycle in response to cAMP waves), aggregation streams, a top view of a mound with incoming streams, a side view of a tipped mound, a side view of a migrating slug and an early culminant and a fruiting body with, on its side, high‐magnification images of the stalk cells and spores. The developmental cycle is starvation induced and takes 24 h at room temperature.

Figure 2.

Regulation of adenylyl cyclase. Cyclic adenosine monophosphate (cAMP) binding to the cAMP receptor R1 (cAR1) induces dissociation of G2 into its α and βγ subunits. The free βγ subunits present a binding site to the cytosolic regulator of adenylyl cylase (CRAC), which induces its translocation to the plasma membrane, where the complex of βγ and CRAC activates adenylyl cyclase (ACA). Activation of ACA also involves three other proteins, aimless, pianissimo and ERK2, that function in ways that are not yet understood. cAMP is rapidly secreted to the exterior, where it augments the response and is ultimately hydrolysed by extracellular phosphodiesterase (PDE). cAMP binding to R1 also initiates an inhibitory response, which may be mediated by an inhibitory G protein. The inhibitory response prevents formation of free βγs and translocation of CRAC.

Figure 3.

Darkfield waves caused by the movement of cells in response to propagating cAMP waves. The top image shows a darkfield image of a monolayer of aggregating cells at low magnification. The cAMP waves propagate as spiral waves. Where they meet they annihilate due to the adaptation of the signalling pathway. The lower diagram shows two cAMP waves propagating from left to right. The bottom line shows how the cells move in response to these waves. As soon as the cells experience an increase in cAMP they round up (in the ‘cringe’ response). In this form they scatter little light and therefore appear dark under darkfield illumination. As soon as they detect an increase in cAMP they start to move up the cAMP gradient (towards the left) and elongate. Actively moving, elongated cells scatter a lot of light and groups of moving cells appear as light bands in the top diagram. As the wave passes and the concentration of cAMP falls, the cells stop moving and return to a more amoeboid shape, which leads to intermediate light scattering or grey bands in the top image. Therefore, the darkfield waves reflect the underlying cAMP waves.

Figure 4.

DIF‐1 signalling. (a) Initial cell type differentiation DIF is produced by the prespore cells and causes uncommitted cells to differentiate as prestalk cells. DIF acts antagonistically to cAMP, repressing further prespore differentiation, one of the immediate responses to DIF is the production of DIFase. This acts to limit the fraction of cells that differentiate as prestalk cells. (b) The structure of DIF‐1.

Figure 5.

Distribution of the prestalk cell types and organization of the ecmA promoter. This figure depicts a Dictyostelium slug and shows the various cell types in a highly schematic form. The cells in the prestalk region are represented as blocks of colour as are the prespore cells. The ALC are shown as individual cells but the drawing is not to scale. There are in reality approximately as many ALC as there are anterior prestalk cells. Below the slug is a representation of the ecmA promoter fragments, one of the promoters used to deduce this organization of cell types. The ecmB promoter, is not shown. It directs strong expression in pstAB cells and weaker expression in pstB cells. The cudA promoter directs expression in prespore and tip‐organizer cells. Here the pstAB and tip‐organizer cells are shown as partially overlapping populations and the pstAB cells are shown as a dotted line because they are found in only some of the slugs within a population.

Figure 6.

Tip‐organizer‐specific gene expression. This figure depicts a proposed pathway for tip‐organizer‐specific gene expression of cudA. The tip‐specific element of cudA contains an imperfect dyad AGATAATCT that is essential for expression and that binds STATa (Fukuzawa and Williams, ). The proposed pathway postulates an elevated cAMP concentration in the tip‐organizer region, caused by the tissue‐specific expression of the ACA adenylyl cyclase. This leads to localized activation of STATa and to consequent expression of cudA.

Figure 7.

Prestalk cell movement and differentiation at culmination. This is a highly diagrammatic representation of the stages in culmination. The ecmA promoter can be divided into two parts, a proximal part (the ecmA region) that directs expression predominantly in cells within the tip (i.e. in the pstA cells) and a distal part (the ecmO region) that directs expression in cells in the back of the prestalk region (the pstO cells) and in a subset of the anterior‐like cells (ALC). Another subset of the ALC, the pstB cells, are defined by selective staining with neutral red and because they express the ecmB gene at a high level relative to the ecmA gene. The pstB cells have a complex movement pattern during slug migration and at culmination they merge with rearguard cells to form the outer part of the basal disc. The pstAB cells express both the ecmA and ecmB genes at a high level. The pstAB cells that make up the stalk are formed at culmination by the differentiation of first the pstA cells and then the pstO cells. The event that marks this differentiation is expression of the ecmB gene. There is also a cone of pstAB cells within the tip of the migrating slug. At culmination these cells migrate down just ahead of the stalk and come to form the inner part of the basal disc but, for the sake of clarity, they are not shown as a separate population in the figure.

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Weijer, Cornelis J, and Williams, Jeffrey G(Dec 2009) Dictyostelium: Cell Sorting and Patterning. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001116.pub2]